The present invention relates to wireless communications in general and, in particular, to certain packet clustering and frame formation techniques in an adaptive coding and modulation (ACM) system. Using ACM, the modulation format and Forward Error Correction (“FEC”) codes for a signal may be adapted to better match the link conditions for each user in a multi-user system. A return channel or other means may be used to report the conditions of a receiving terminal. In a broadcast system, for example, the waveform broadcast to a number of users includes data packets designated only for an individual terminal (or small group of terminals). A message transmitted to a user requires fewer symbols (and thus less time) when a higher order modulation and higher code rate are used. Lower order modulation and lower code rate are more reliable but require more time to transmit the same amount of data. Using ACM, each packet may be transmitted at an optimized modulation and coding format (“modcode”) level given the destination terminal's link conditions.
In optimizing modcode levels for a destination terminal's link conditions, first-in first-out (FIFO) queueing is used in some traditional systems. However, different sets of packets may have different modcodes and have varying quality of service requirements. In some implementations, there can be many packets in a single frame being transmitted according to a given modcode. Using certain traditional methods may be inefficient, however, because the frame formation may result in additional padding or header overhead, or cause delay. It may, therefore, be desirable for there to be novel techniques for forming frames to be transmitted.